Re: [PATCH 0/4] cover letter: qemu: Implement support for iommufd

[Ján Tomko via Devel]

On a Thursday in 2025, Nathan Chen wrote:

On 11/6/2025 10:14 AM, Ján Tomko wrote:

Hi,

This series implements support for using iommufd to propagate DMA
mappings to the kernel for VM-assigned host devices in a qemu VM.

We add a new 'iommufd' attribute for hostdev devices to be
associated with the iommufd object.

For instance, specifying the iommufd object and associated hostdev in a
VM definition:

 <devices>
...
   <hostdev mode='subsystem' type='pci' managed='no'>
     <driver iommufd='yes'/>
     <source>
       <address domain='0x0009' bus='0x01' slot='0x00' function='0x0'/>
     </source>

   </hostdev>
   <hostdev mode='subsystem' type='pci' managed='no'>
     <driver iommufd='yes'/>
     <source>
       <address domain='0x0019' bus='0x01' slot='0x00' function='0x0'/>
     </source>

   </hostdev>
...
 </devices>

     <address type='pci' domain='0x0000' bus='0x15' slot='0x00' function='0x0'/>      <address type='pci' domain='0x0000' bus='0x16' slot='0x00' function='0x0'/>

Are there any hardware/kernel requirements, or something done to be
host? Even when I add the device to the vfio-pci driver, it does not
create /dev/vfio/devices for me:



Kernel: 6.17.6-300.fc43.x86_64 on Fedora

error: unsupported configuration: VFIO device /dev/vfio/devices/vfio0 not found - ensure device is bound to vfio-pci driver

The answer was that the Fedora kernel did not have
CONFIG_VFIO_DEVICE_CDEV=y
enabled.

Jano

QEMU: v10.1.0-2147-g917ac07f9a (the current master)

Also, I'd expect it to just work with managed='yes'.

# lsmod | grep iommufd
iommufd               327680  1 vfio



-Nathan

The iommufd module should be loaded before adding the device to the vfio-pci driver: I will ensure the iommufd module gets loaded with managed='yes' in the next revision, thanks for catching that.

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